Study On Energy Density Performance And Interface Control Of PVDF Based Dielectric Materials

The studying object of this paper was a two-phase dielectric nanocomposites system--BaTiO₃//PVDF organic-inorganic dielectrics.The energy density performance of polymer based dielectrics was controlled through experimental and simulated method by the interface structure and properties of the nanocomposites,such as surface functionalization of BaTiO₃/ powder and PVDF matrix,differences of interface conductivity,physical binding ways between BaTiO₃/ particles and PVDF molecular,etc.First,experiment was carried out from the perspective of surface functionalization and morphology design of BaTiO₃/ particles in order to obtain BaTiO₃//PVDF nanocomposites with high energy density.Then finite element method was used to simulate the dielectric performance of the nanocomposites.The paper found the relationship between BaTiO₃/ contents,structure parameters and the dielectric performance of the nanocomposites,which could guide the experimental preparation.Next,we simulated the interface interaction and charges distribution of the nanocomposites and found the enhanced mechanism of the energy density.Finally,conductive carbon-carbon double bonds were formed in the PVDF molecular,which could realize the structure functionalization of organic phase and improve the energy density performance of PVDF matrix.The detailed contents were listed as follows.Spherical BaTiO₃/ particles with the size of 50 nm and 500 nm were surface-functionalized with polarized-C=O groups.The modifying agents included ethyl p-nitrobenzoate?EPNB?,cis-butenedioic acid?cis-BA?,ethyl p-aminobenzoate?EPAB?,etc.The experimental results found that the functionlization effect of cis-BA and EPNB was sufficient.The dielectric constant and energy density of 50 nm EPNB@BaTiO₃//PVDF nanocomposites at 1 kHz were 12.7 and 2.0 J/cm3,respectively,which were 5%and 46%higher compared with untreated materials.The dielectric constant and energy density of 500 nm cis-BA@BaTiO₃//PVDF nanocomposites at 1 kHz were 13.6 and 3.6 J/cm3,respectively,which were 28%and 125%higher compared with untreated materials.These results indicated that-C=O groups on the surface of BaTiO₃/ particles could enhance interface polarization of the nanocomposites.Molecular dynamic simuation results showed that interface van der waals force of EPNB@BaTiO₃//PVDF and cis-BA@BaTiO₃//PVDF nanocomposites increased,which were 33%and 19%higher compared with BaTiO₃//PVDF nanocomposites.The interface charges of these two composites were discontinuously distributed.These results showed that the surface functionalization of inorganic particles could enhance interface interaction and distribute interface charges.Therefore,the energy density performance of the nanocomposites was enhanced.BaTiO₃/ nanowires,nanoflakes and microrods were prepared by solvothermal-ionic exchange method from the view of designing and synthesizing multi-morphological BaTiO₃/ particles.These inorganic powders were used as fillers to fabricate BaTiO₃//PVDF nanocomposites.The energy density performance of nanocomposites was studied.Compared with BaTiO₃/ nanoflakes/PVDF and BaTiO₃/ microrods/PVDF nanocomposites,BaTiO₃/ nanowires/PVDF nanocomposites possessed better insulating properties.The breakdown strength of 3 vol%BaTiO₃/ nanowires/PVDF nanocomposites was 417 kV/mm,which was 2.1 and 2.5 times that of BaTiO₃/ nanoflakes/PVDF and BaTiO₃/ microrods/PVDF nanocomposites with the same BaTiO₃/ content,respectively.When BaTiO₃/ content increased to 10 vol%,the breakdown strength decreased to 344 kV/mm,but the maximum energy density reached to 8.1 J/cm3.Modifying agents cis-BA and EPNB were also used to surface functionalized BaTiO₃/ nanowires.The energy density of treated 3 vol%PVDF based nanocomposites was 14.6 J/cm 3 and 11.7 J/cm3,respectively.The size of BaTiO₃/ nanowires was small and the surface radian of BaTiO₃/ nanowires matched well with the radian of flexible PVDF molecular.Moreover,the hydroxyl groups on the surface of BaTiO₃/ nanowires could form hydrogen bonds with the F atoms in PVDF molecular.The combined effects make PVDF molecular wrap on the surface of BaTiO₃/ nanowires.This combination mode strengthens the physical binding interaction between BaTiO₃/ nanowires and PVDF molecular.Free charges were restricted in the interface and the energy density of the nanocomposites was enhanced.The dielectric performances of BaTiO₃//PVDF nanocomposites were simulated by multi-field coupling software COMSOL.As BaTiO₃/ particle size became smaller,the intensity of average interface electric field decreased and the insulting performance of the nanocomposites enhanced.As BaTiO₃/ contents increased,the dielectric constant increased while the insulting performance became poor.The size of porous BaTiO₃/ particles was fixed to 50 nm.As the porosity and pore diameter of porous BaTiO₃/ particles increased,the dielectric constant of porous BaTiO₃//PVDF nanocomposites rised and the insulating performance enhanced first and then became poor.When the porosity was 60%and pore diameter was 16 nm,the nanocomposites possessed best insulating performance.The results showed that optimized pore structure parameters exsited for best dielectric performance of BaTiO₃//PVDF nanocomposites.Finally,part of H and F atoms in PVDF molecular were removed by elimination reaction with KOH solution in order to form conductive carbon-carbon double bonds and realize the control of organic PVDF phase.In KOH solution,PVP was treated as surfactant and dehydrofluorinated PVDF?d-PVDF?dielectrics containing polyene structure were prepared.The experiment studied the effect of reaction temperature,time and concentration of KOH solution on the energy density performance of d-PVDF dielectrics.As the increase of reaction temperature,reaction time and the concentration of KOH aqueous solution,the dielectric constant decreased first and then increased,the breakdown strength and energy density showed opposite change tendencies.Under the modification circumstances of 80?,6 h and 0.81 mol/L KOH aqueous solution,the dielectric constant and loss of d-PVDF dielectrics were 10.8 and 0.024,the conductivity was 13.8×10-9 S/m,the breakdown strength reached 452 kV/mm and the maximum energy density was 10.5 J/cm3.The energy density and conductivity of d-PVDF dielectrics were four times and twice those of unmodified PVDF,respectively.Electrons in conjugated ? bonds of polyene structure could be accelerated to transfer under the applied electric field.Therefore,the conductivity of modified PVDF could be enhanced.The composition of d-PVDF dielectrics included PVDF molecular and modified PVDF molecular.PVDF molecular formed the continuous phase and modified PVDF molecular formed the discontinuous phase.The proper conductivity difference between PVDF molecular and polyene molecular could induce Maxwell-Wagner effects to enhance energy density performance of d-PVDF dielectrics without increasing dielectric loss at the same time.Thus practicability of d-PVDF dielectrics was guaranteed.